1. Field of the Invention
The present invention relates to a smoke detecting apparatus for a fire alarm which includes a light emitting lamp, such as a xenon lamp, and a light receiving element provided in a smoke chamber so as to detect the smoke density in the smoke chamber in accordance with an output signal of the light receiving element upon receiving light emitted from the light emitting lamp and scattered by the smoke inside the smoke chamber.
2. Description of the Related Art
A conventional smoke detecting apparatus, which detects the smoke density in the smoke chamber in accordance with an output signal of a light receiving element upon receiving light from a xenon lamp, is unable to operate normally after a certain length of service because the xenon lamp loses its light emitting efficiency or is no longer able to emit light at all after a certain number of service years or a certain number of light emissions.
To detect the failure of the xenon lamp, the known smoke detecting apparatus monitors the reception level of the light from the xenon lamp. When the reception level falls below a predetermined level, the apparatus determines that the xenon lamp has failed. More specifically, when no reception above a predetermined level has been detected in a predetermined length of time, for example, 30 or 60 seconds, the apparatus determines that the xenon lamp has failed, and indicates the failure of the xenon lamp by, for example, producing an audible-alarm.
Although the conventional apparatus is able to indicate that the xenon lamp has failed, it is unable to predict the lamp failure. Therefore, when the xenon lamp of the conventional smoke detecting apparatus has failed, the smoke detecting apparatus remains out of operation for some time before a maintenance person notices the failure and replaces the xenon lamp. Such an inoperative period is undesirable. Other types of smoke detecting apparatuses employing light emitting lamps other than the xenon lamp are also subject to the same problem.